Earth's inner core, made up of solid iron, 'superrotates' in an
eastward direction -- meaning it spins faster than the rest of the
planet -- while the outer core, comprising mainly molten iron, spins
westwards at a slower pace.
Although Edmund Halley -- who also discovered the famous comet --
showed the westward-drifting motion of Earth's geomagnetic field in
1692, it is the first time that scientists have been able to link the
way the inner core spins to the behavior of the outer core. The planet
behaves in this way because it is responding to Earth's geomagnetic
field.
The findings, published today in Proceedings of the National Academy of Sciences, help scientists to interpret the dynamics of the core of Earth, the source of our planet's magnetic field.
In the last few decades, seismometers measuring earthquakes
travelling through Earth's core have identified an eastwards, or
superrotation of the solid inner core, relative to Earth's surface.
"The link is simply explained in terms of equal and opposite action,"
explains Dr Philip Livermore, of the School of Earth and Environment at
the University of Leeds. "The magnetic field pushes eastwards on the
inner core, causing it to spin faster than Earth, but it also pushes in
the opposite direction in the liquid outer core, which creates a
westward motion."
The solid iron inner core is about the size of the Moon. It is
surrounded by the liquid outer core, an iron alloy, whose
convection-driven movement generates the geomagnetic field.
The fact that Earth's internal magnetic field changes slowly, over a
timescale of decades, means that the electromagnetic force responsible
for pushing the inner and outer cores will itself change over time. This
may explain fluctuations in the predominantly eastwards rotation of the
inner core, a phenomenon reported for the last 50 years by Tkalčić et
al. in a recent study published in Nature Geoscience.
Other previous research based on archeological artefacts and rocks,
with ages of hundreds to thousands of years, suggests that the drift
direction has not always been westwards: some periods of eastwards
motion may have occurred in the last 3,000 years. Viewed within the
conclusions of the new model, this suggests that the inner core may have
undergone a westwards rotation in such periods.
The authors used a model of Earth's core which was run on the giant
super-computer Monte Rosa, part of the Swiss National Supercomputing
Centre in Lugano, Switzerland. Using a new method, they were able to
simulate Earth's core with an accuracy about 100 times better than other
models.